1. Field of the Invention
The invention relates to temperature-variable microwave attenuators.
2. Description of the Related Art
Attenuators are used in applications that require signal level control. Level control can be accomplished by either reflecting a portion of the input signal back to its source or by absorbing some of the signal in the attenuator itself. The latter is often preferred because the mismatch which results from using a reflective attenuator can create problems for other devices in the system such as nonsymmetrical two-port amplifiers. It is for this reason that absorptive attenuators are more popular, particularly in microwave applications. The important parameters of an absorptive attenuator are: attenuation as a function of frequency; return loss; and stability over time and temperature.
It is known that variations in temperature can affect various component parts of a microwave system causing differences in signal strengths at different temperatures. In many cases, it is impossible or impractical to remove the temperature variations in some Radio Frequency (RF) components. For example, the gain of many RF amplifiers is temperature dependent. In order to build a system with constant gain, a temperature-dependent attenuator is placed in series with the amplifier. The attenuator is designed such that a temperature change that causes the gain of the amplifier to increase will simultaneously cause the attenuation of the attenuator to increase such that the overall gain of the amplifier-attenuator system remains relatively constant.
Prior art temperature-dependent attenuators employ connections between an unbalanced transmission line and ground or between two lines of a balanced line. Such constructions is not always optimal, especially at frequencies above 18 GHz. The reason for this is that parasitic capacitances and inductances can taint (or alter) the response of the device so that its attenuation over frequency and VSWR is no longer useful or desirable. It is typically desirable that the attenuation at any particular temperature is constant over the frequency of interest and the VSWR is as low as possible, usually less than 2.0 to 1. For frequencies exceeding 18 GHz, the prior art is unable to achieve this with any degree of accuracy. Therefore there is a real need for a temperature dependent attenuator that exhibits flat (or nearly flat) attenuation characteristics and low VSWR over selected portions of the frequency range of 18 GHz to 300 GHz.
The present invention solves these and other problems by providing a temperature-dependent attenuator that uses two or more temperature-dependent resistors in series with a transmission line. The attenuator can be used at radio frequencies, microwave frequencies, etc. In one embodiment, the temperature-dependent radio-frequency attenuator includes a plurality of temperature-dependent resistors electrically in series with a transmission line. The temperature-dependent resistors are in series with the transmission line approximately one-quarter wavelength apart at a desired frequency. The temperature coefficients of the resistors are configured such that the attenuator changes attenuation at a desired rate with changes in temperature.
In one embodiment, the resistors have different temperature coefficients. In one embodiment, the resistors have the same, or similar, temperature coefficients. In one embodiment, temperature coefficient of one or more of the resistors is a negative temperature coefficient of resistance. In one embodiment, temperature coefficient of one or more of the resistors is a positive temperature coefficient of resistance. In one embodiment, one or more of the resistors are film resistors. In one embodiment, one or more of the resistors are thick-film resistors. In one embodiment, one or more of the resistors are thin-film resistors. In one embodiment, one or more of the resistors are printed ink resistors.
In one embodiment, the attenuator has a negative temperature coefficient of attenuation. In one embodiment, the attenuator has a positive temperature coefficient of attenuation. In one embodiment, the transmission line includes a microstrip transmission line. In one embodiment, the transmission line includes a stripline transmission line.
In one embodiment, the attenuator has three resistors in series with the transmission line and approximately one-quarter wavelength apart. In one embodiment, the middle resistor in the series has a resistance that is approximately twice the resistance of the outer. In one embodiment, the two outer resistors have approximately the same resistance.
In one embodiment, an attenuator includes a series combination of temperature-dependent resistors separated by transmission line sections. In one embodiment, the transmission line sections have different lengths and/or characteristic impedances. In one embodiment, the lengths of the transmission line sections are symmetric about an electrical center of the attenuator.
In one embodiment, the attenuator uses a microstrip transmission line. In one embodiment, the attenuator uses a stripline transmission line. In one embodiment, the attenuator uses a co-planer waveguide transmission line. In one embodiment, the attenuator uses a grounded co-planar waveguide transmission line. In one embodiment, the attenuator uses a coaxial transmission line. In one embodiment, the VSWR of the attenuator remains below 3 to 1 over a desired operating band.